JP5392591B2 - Electrowetting display device - Google Patents

Description

  The present invention provides at least one first liquid that is immiscible with each other on a first transparent substrate and a second liquid that is electro-conductive or polar. The present invention relates to a display device.

  In general, the liquid is contained in the space between the first support substrate and the second support substrate, but this is not strictly necessary.

  If the liquid is oil (colored) (due to the tension between the two sides) and the second liquid is water, a two-layer system with a water layer and an oil layer is provided. However, when a voltage is applied between the water and the electrode of the first support substrate, the oil layer moves aside or separates due to electrostatic forces. The water portion penetrates into the oil layer, and the pixel becomes partially transparent.

  A display device based on this principle is described in PCT application WO03 / 00196 (PH-NL02.129).

  An intermediate optical state or gray level is an optical state between a first extreme optical state and a second extreme optical state (eg, full black and full white), wherein the extreme optical state It can be achieved using a DC voltage level intermediate between the voltage levels that introduce the state (eg, zero and maximum voltage level). In practice, however, the resulting gray scale stability or reproducibility is very poor and is empirically found to be unacceptable in most applications.

  One of the objects of the present invention is to provide a display having drive means that provides acceptable gray scale stability.

  In order to achieve the above object, the display device according to the present invention applies a voltage to the electrode related to the range of the electro-optical state of the pixel between the first extreme state and the second extreme optical state. Driving means for providing a variable voltage to the pixel during the selection of the pixel; The variable voltage may be an alternating voltage having an average value substantially equal to the voltage associated with the electro-optical state of the pixel being set. The variable voltage may have a DC portion and an AC portion, and the maximum and minimum voltages of the alternating voltage are average square roots that are substantially equal to the voltage associated with the electro-optical state of the pixel to be set. Have.

  In a further embodiment, such driving means supplies a preceding voltage to the pixel before said voltage associated with the electro-optical state. For example, the preceding voltage includes a set of alternating voltages having an average value substantially equal to zero or an average value substantially equal to the voltage associated with the electro-optical state of the pixel to be set.

  The present invention improves the homogeneity of the oil film switching behavior by applying these preceding voltages, avoids local diffusion of the oil film and reduces the possibility of oil film failure.

  In another embodiment, the display device comprises drive means for applying a voltage to the electrodes associated within the range of the electro-optic state of the pixel between the first extreme state and the second extreme state. The driving means supplies a voltage to the pixel before selecting the pixel to put the pixel in one of the extreme states.

  These and other aspects of the invention will be apparent with reference to non-limiting embodiments. The drawings are conceptual and corresponding elements that are not drawn for scaling are generally indicated by the same reference numerals.

  FIG. 1 shows a conceptual cross section of a part of a display device 1 which shows the principle on which the display device according to the invention is used. Between the two transparent substrates or support plates 3 and 4, a first liquid 5 and a second liquid 6 are provided, which are immiscible with each other. The first liquid 5 is, for example, an alkane such as hexadecane or (silicon) oil in this example. The second liquid 6 is conductive or polar, for example water or an aqueous salt solution (for example, an aqueous solution of potassium chloride in a mixture of water and ethyl alcohol).

  In the first state, when no external voltage is applied (FIG. 1a), the liquids 5, 6 are adjacent to the first or second transparent support plate 3, 4 of eg glass or plastic and the first support plate 3 For example, a transparent electrode 7 made of, for example, indium (tin) oxide, an intermediate non-wetting (hydrophobic) layer 8 made of an amorphous fluoropolymer (AF 1600) in this example is provided.

  When a voltage is applied via the interconnects 20, 21 (voltage source 9), the layer 5 moves aside or breaks up into small drops (FIG. 1b). This occurs when the electrostatic energy gain is greater than the loss of surface energy due to the creation of a curved surface. As a very important aspect, reversible switching between the continuous film 5 covering the support plate 3 and the film adjacent to the wall 2 is achieved by electrical switching means (voltage source 9). The

  FIG. 2 is an electrical equivalent circuit diagram of a part of the apparatus 1 to which the present invention is applied. One possible embodiment (one drive mode called “passive mode”) has a matrix of pixels 18 at the intersection of the row or select electrode 17 and the column or data electrode 16. The row electrodes are continuously selected by the row driver 14, and data is supplied to the column electrodes via the data register 15. Thus, incoming data 22 is first processed by the processor 13 when necessary.

  Mutual synchronization between the row driver 14 and the data register 15 is performed via a drive line 19. The selection electrode 17 and the data electrode 16 are either directly through individual electrodes as shown by the electrodes 20, 21 in FIG. 1, or a threshold element such as a non-linear resistance or a non-linear switching element such as a MIM or a diode. For example, it is connected to the liquids 5 and 6. A row of pixels 18 may be driven by one or more select electrodes 17, and similarly, a column of pixel electrodes may be driven by one or more data electrodes 16.

  In another possible embodiment (another driving mode called “active mode”), the signal from the row driver 14 selects the pixel electrode through a thin film transistor (TFT) 30, and the gate electrode of the TFT is the column The source electrode is electrically connected to the column electrode. A signal present in the column electrode 16 is sent to the pixel electrode of the pixel 18 coupled to the drain electrode via the TFT. The other pixels are connected to, for example, one (or more) common counter electrode. In FIG. 2, only one thin film transistor (TFT) 30 is simply illustrated as an example. Other “active mode” configurations are alternatively possible. Further, one or more select electrodes 17 may drive a row of TFTs 30 and one or more data electrodes 16 drive a column.

FIG. 3 shows a first pulse pattern scheme with four prepulses (alternating voltage) 31 preceding the fixed voltage 32 (with a selection period t _SEL ) for the display. The value of the fixed voltage 32 (V ₁ , V ₂ , V ₃ ) determines the gray value. The pulse length of the pre-pulse is not limited, but is preferably on the order of magnitude less than the minimum time period required to drive the display from a full black state to a full white state. For example, the maximum voltage level V _max that can be used in the driver IC is preferably used as the amplitude of the pre-pulse 31. The number of these alternating pre-pulses in the gray scale transition is arbitrarily selected but is preferably even, but the overall time period of these alternating pulses in the gray scale transition is 50% of the selected period t _sel . Is preferably smaller. Gray level accuracy and stability are improved after using a series of short AC pulses prior to the gray level drive pulse before each transition. In this example, four short pulses are used, two negative pulses and two positive pulses, and the average DC voltage is equal to zero.

FIG. 4 conceptually shows a further pulse pattern, in which a series of short pre-pulses 31 are provided prior to the grayscale drive voltage pulse 32 for each transition. In this example, four short pulses, two negative pulses and two positive pulses, are used, and the average DC voltage is equal to the gray level voltages V ₁ , V ₂ , V ₄ . The prepulse is extremely high to improve gray scale reproducibility in electro-wetting displays. However, the prepulse produces an optical response that is sometimes visible when a long prepulse is used. In order to reduce the optical disturbances induced by this prepulse, the prepulse is applied to different parts of the screen with different polarities.

  In the following frames, the positive polarity and the negative polarity are reversed. Since the eye averages these short range brightness variations over subsequent frames, the perceptual overview of the display is hardly affected.

  Another possible pulse pattern is conceptually illustrated in FIG. 5, where the prepulse 31 has a predetermined amplitude at the beginning of the prepulse sequence and a small amplitude (prepulse 31 ′) at the end of the prepulse sequence. Have. The voltage sweeps between a negative voltage decrease and a positive voltage decrease, resulting in low power consumption (especially with a very large number of prepulses).

  FIG. 6 illustrates a fourth embodiment, where the AC pulse has a reduced pulse time period and constant amplitude. The advantage of this embodiment is that the update time of the entire image is shortened. The advantage of this embodiment is that the overall image update time is shortened. In a combined embodiment (not shown), the prepulse has both a variable amplitude and a variable pulse time period. This will provide additional flexibility for optimal combining, resulting in minimal image update time, minimal power consumption, low light flicker and optimal performance.

In addition, improved gray scale reproducibility and relatively short image update time can be obtained by changing the gray scale drive voltage pulse 32 to a series of short voltage pulses 33 with alternating amplitudes. Is equal to or close to the DC voltage levels V ₁ , V ₂ required to drive the display to the desired gray level. Although not shown, the short voltage pulse 33 may be preceded by a pre-pulse, similar to the pulses in the embodiments of FIGS.

FIG. 8a shows the root mean square (RMS) average voltage √ [(V _max ² + V _min ² ) / 2 approximately equal to the DC voltage level required to drive the display to the desired gray level V _1. , And FIG. 8b shows an asymmetric time period with an RMS average voltage approximately equal to the DC voltage level required to drive the display to the desired gray level. A series of short voltage pulses is shown.

  FIG. 9 shows a reset pulse 35 that puts a pixel into one of the extreme states (either completely on or completely off) before applying a grayscale drive pulse 36 for a grayscale transition. Used to show how to drive the display. The length of the preset pulse is not limited, but is preferably as short as possible, for example, less than 50% of the total addressing time, depending on the application and the characteristics of the display element. Since the response time and the resulting reset depends on the previous gray value, the reset pulse has a reset voltage and a pulse width (total pulse energy) to reset all possible gray values to the initial state.

  In the case where the reset characteristics of the display elements (especially response time) require a longer reset time, in matrix driving, the reset pulse is a predetermined period before the selection time for providing information to the display elements in the same row. May be given during the selection time of the row of display elements (may be applied during the previous frame time).

  The present invention is not limited to the examples described above. If there is a difference between the switch-on speed and the switch-off speed, the RMS requirements of FIGS. 7 and 8 may not give the correct gray value. The average voltage level during these short pulses is selected so that the resulting reflection vs. time curve fits the gray scale drive voltage vs. time curve. Then, the gray level does not drift, the image update time remains short, and the image quality is significantly improved. The use of asymmetric pulses allows the pulses to be accurately adjusted to achieve an accurate gray scale.

  Some of the displays may be driven in other drive modes other than other parts, and in a given mode, the frequency, voltage value, and polarity of the aforementioned voltages may be different for different parts of the display.

  Although a display device between two transparent substrates is shown, the support plate 7 with the electrode 7 and the oil layer 5 (FIG. 1a) may even be immersed in an open water container.

  Each of the present invention lies in new features and combinations thereof. Reference numerals in the claims do not limit their protective scope. The use of the verb “comprise” and its derivatives does not exclude the presence of elements other than those listed in a claim. Use of the article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.

It is a figure which shows notionally some cross sections of the display apparatus in which this invention is used. It is a figure which shows notionally the electrical equivalence of a display apparatus. It is a figure which shows notionally the method of driving a pixel with the apparatus which concerns on this invention. FIG. 6 conceptually illustrates another way of driving a pixel with the device according to the present invention. It is a figure which shows notionally the method of driving a pixel with another apparatus based on this invention. FIG. 6 conceptually illustrates another way of driving a pixel with the device according to the present invention. FIG. 6 conceptually illustrates another way of driving a pixel with the device according to the present invention. It is a figure which shows the change of FIG. 7 notionally. FIG. 6 conceptually illustrates another method of driving a pixel with an apparatus according to the present invention using a reset method.

Claims (8)

A display device having at least one pixel having an optical switch comprising at least one first liquid that is immiscible with each other on the first support plate and a second liquid that is conductive or polar,
The display device has driving means for applying a voltage related to a range of the electro-optical state of the pixel between the first extreme state and the second extreme state to the electrode of the optical switch;
The driving means supplies an alternating voltage with varying amplitude or period to the pixel during selection of the pixel before supplying a voltage related to the electro-optical state;
The variable alternating voltage includes a set of alternating voltages having an average value substantially equal to a voltage associated with the electro-optical state of the pixel to be set.
A display device characterized by that.   The display device according to claim 1, wherein a liquid is contained in a space between the first transparent support plate and the second support plate. A display device having at least one pixel having an optical switch comprising at least one first liquid that is immiscible with each other on the first support plate and a second liquid that is conductive or polar,
The display device has driving means for applying a voltage related to a range of the electro-optical state of the pixel between the first extreme state and the second extreme state to the electrode of the optical switch;
The driving means supplies an alternating voltage with varying amplitude or period to the pixel during selection of the pixel before supplying a voltage related to the electro-optical state;
The alternating voltage of varying amplitude or period comprises a set of alternating voltages having an average voltage substantially equal to zero. The display device according to claim 3 , wherein in driving at least one pixel, an amplitude of an alternating voltage whose amplitude or period changes is decreased. The display device according to claim 3 , wherein in driving at least one pixel, the frequency of the alternating voltage whose amplitude or period changes increases. 4. A display device according to claim 3 , wherein the alternating voltage of varying amplitude or period has a different value for different parts of the display. 4. A display device according to claim 3 , wherein the alternating voltage of varying amplitude or period has a different polarity for different parts of the display. The display device according to claim 1, wherein the alternating voltage whose amplitude or period changes includes a voltage to the pixel that causes the pixel to be in one of the extreme states.

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